Engine miss - diagnosis and solution
03-03-2007, 02:37 AM
#1
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Engine miss - diagnosis and solution
I thought I'd share the diagnosis and solution to my '87 S4 idle miss that was cured today. I hope it may help someone in the future.
Background: within last 8 months/10K miles, new rotors, spark plug wires, spark plugs when cam belt retension was done. Was running great at this time.
Symptoms: over time, the engine idling became more rough until the car wasn't driveable (102K miles). It would run fine when cold, would drive smooth when warm and driven at over 1,000 RPM.
Steps to diagnosis, items ok or no change in bad running after all of these items ......
- check temp II sensor
- check idle speed sensor (OK for idle, high load not working)
- check exhaust, #1 cylinder side Hydro Carbon top of scale (lean miss), #2 cylinder side running a little rich ... this is a good reason NOT to remove the CO test tubes, an exhaust test yields valuable information.
- swap MAF, LH, EZK with known good units
- suspected clogged fuel injector(s). Remove injectors, test showed bad flow pattern, flow about 10% variance. Clean and replace screens, all injectors now good. Didn't fix the problem (injectors needed cleaning anyway).
- one by one remove spark plug wires, HC goes down when #1 removed.
- one by one remove fuel injection wire, HC goes down when #1 removed
>> attention now focused to number one cylinder, something strange with combustion
- change spark plug
- change fuel injector
- change spark plug wire
- swap rotors
- swap rotor caps
>>> no improvement
- compression test/leak down test, 200 lbs compression, less that 3% loss on leak down ... very good
>>> At this point, verified spark is good, injection is good, compression is good. All the normal suspect items eliminated
Final test, smoke test ... force pressurized smoke into the intake manifold.
Result, leak found on inside of #1 cylinder intake gasket! This does explain all of the symptoms. Cold start enrichment was enough that the engine would run smoothly when cold. Less vacuum and higher flow rate means less of a lean miss condition when engine is running at higher RPM. The HCs were high on the side of the engine with the vacuum leak (too lean).
So, the intake manifold had to be removed and the gasket was replaced. I opted for the complete while you're in there fixes - replace knock detecors, throttle position switch, fuel lines, flappy actuator, flappy bearings, etc.
Car runs perfect. Work done by Exotic Motorwerks. Looking forward to another happy 100,000 miles. The only other major service planned is to replace the cam chains at around 125,000 miles.
Background: within last 8 months/10K miles, new rotors, spark plug wires, spark plugs when cam belt retension was done. Was running great at this time.
Symptoms: over time, the engine idling became more rough until the car wasn't driveable (102K miles). It would run fine when cold, would drive smooth when warm and driven at over 1,000 RPM.
Steps to diagnosis, items ok or no change in bad running after all of these items ......
- check temp II sensor
- check idle speed sensor (OK for idle, high load not working)
- check exhaust, #1 cylinder side Hydro Carbon top of scale (lean miss), #2 cylinder side running a little rich ... this is a good reason NOT to remove the CO test tubes, an exhaust test yields valuable information.
- swap MAF, LH, EZK with known good units
- suspected clogged fuel injector(s). Remove injectors, test showed bad flow pattern, flow about 10% variance. Clean and replace screens, all injectors now good. Didn't fix the problem (injectors needed cleaning anyway).
- one by one remove spark plug wires, HC goes down when #1 removed.
- one by one remove fuel injection wire, HC goes down when #1 removed
>> attention now focused to number one cylinder, something strange with combustion
- change spark plug
- change fuel injector
- change spark plug wire
- swap rotors
- swap rotor caps
>>> no improvement
- compression test/leak down test, 200 lbs compression, less that 3% loss on leak down ... very good
>>> At this point, verified spark is good, injection is good, compression is good. All the normal suspect items eliminated
Final test, smoke test ... force pressurized smoke into the intake manifold.
Result, leak found on inside of #1 cylinder intake gasket! This does explain all of the symptoms. Cold start enrichment was enough that the engine would run smoothly when cold. Less vacuum and higher flow rate means less of a lean miss condition when engine is running at higher RPM. The HCs were high on the side of the engine with the vacuum leak (too lean).
So, the intake manifold had to be removed and the gasket was replaced. I opted for the complete while you're in there fixes - replace knock detecors, throttle position switch, fuel lines, flappy actuator, flappy bearings, etc.
Car runs perfect. Work done by Exotic Motorwerks. Looking forward to another happy 100,000 miles. The only other major service planned is to replace the cam chains at around 125,000 miles.
03-03-2007, 08:33 AM
#2
Craic Head
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Rich,
That's the reason this list is so valuable. Your thorough logging of identifying the problem, isolating and eliminating the possible causes and finally finding and fixing it were very thorough. A lesson in methodical technique in general as well as a great write-up on this specific problem.
This is the absolute opposite of the part swapping technique you often find with local mechanics and shade-tree mechanics alike. Part swapping might have fixed it at some point (when the intakes were removed to 'take a look' the gasket would have been replaced) but the real cause would never have been known.
Thanks for this write-up.
That's the reason this list is so valuable. Your thorough logging of identifying the problem, isolating and eliminating the possible causes and finally finding and fixing it were very thorough. A lesson in methodical technique in general as well as a great write-up on this specific problem.
This is the absolute opposite of the part swapping technique you often find with local mechanics and shade-tree mechanics alike. Part swapping might have fixed it at some point (when the intakes were removed to 'take a look' the gasket would have been replaced) but the real cause would never have been known.
Thanks for this write-up.
03-03-2007, 10:53 AM
#3
Rennlist Member
Many, many years ago my father used to tell me there were 2 types of mechanics.
One group he referred to as 'fitters' where they fitted new or different parts until they found the problem. If they were lucky they fitted a part where the problem was. If they weren't lucky they consumed many parts and the customer had a large bill with some long sob story.
The second group he referred to as true mechanics that had the ability to diagnose the actual problem, effect repair and return the vehicle to a good strong running state.
This thread demonstrates the commitment of a true mechanic that not only did the job to the degree required has the integrity to document and share the experience.
Well done Rich.
One group he referred to as 'fitters' where they fitted new or different parts until they found the problem. If they were lucky they fitted a part where the problem was. If they weren't lucky they consumed many parts and the customer had a large bill with some long sob story.
The second group he referred to as true mechanics that had the ability to diagnose the actual problem, effect repair and return the vehicle to a good strong running state.
This thread demonstrates the commitment of a true mechanic that not only did the job to the degree required has the integrity to document and share the experience.
Well done Rich.
Last edited by the flyin' scotsman; 03-03-2007 at 11:37 AM. Reason: spelling
03-03-2007, 11:39 AM
#4
Rennlist Member
we have had this problem (scot and I) on our 928s. a quick way , and one of the first things we do now, is spay chemtool or gumout around the intake. if the rpms flutter or run higher, you have a leak. learned that trick 15 years ago! sorry i didnt mention it earlier
nice job traking the problem down
mk
nice job traking the problem down
mk
